Recently, attention is drawn to optical disks, particularly a digital video disk (DVD) that records a moving image and a sound in a digital system.
Generally, the optical disk e.g. a DVD requiring a large capacity has a structure so-called as a lamination type optical disk which includes, as shown in a schematic view of an optical disk 30 in FIG. 2, two disk substrates 24 having two transparent plastic substrates 21, each being formed on its one side with a recording and reflective layer 22 having formed with a concave and a convex (pits) corresponding to recording information and a protective layer 23 successively in this order, the disk substrates facing each other such that their recording layers (reflective layers) 22 are arranged inside and integrated via an adhesive layer 25.
Adhesives such as a hot-melt type adhesive or a radical polymerizable ultraviolet-curing adhesive have been used as the adhesive layer 25 for bonding the disk substrates 24, but they do not suffice for practical purposes in respect of thermal resistance and disk base warpage.
Under the circumstances, a method of manufacturing an optical disk is proposed as described in e.g. Japanese laid-open Publication No. 7-126577 in which a liquid cationic polymerizable, ultraviolet ray-curing adhesive layer is formed on a bonding face between 2 disks, irradiated with energy rays to effect the bonding and then two disks are bonded with pressure, followed by solidification of the layer.
Such a cationic polymerizable, ultraviolet-curing adhesive layer will initiate curing reaction whether in the air or in a dark place upon exposure to ultraviolet rays once and hence it is expected to have an significant effect on the improvement of the manufacturing process. The cationic polymerizable, ultraviolet-curing adhesive has the advantage of less warpage of disk substrates by virtue of less shrinkage at the time of the reaction, and the adhesive layer after curing becomes extremely hard.
However, even in cases where such cationic polymerizable, ultraviolet-curing adhesives are used, the resulting products cannot suffice under the circumstances of high temperature and high humidity.
For example, if the above-mentioned optical disk is stored under the circumstances of high temperature and high humidity, a tilt angle (i.e. angle from a horizontal plane) of the optical disk is enlarged due to distortion or warpage of the disk, causing an adverse effect on the reproduction of a recorded signal so that the recorded information cannot correctly be read out.
This is particularly a great problem for a DVD requiring high reading accuracy because a concave and a convex (pits) on the recording layer have been further minimized for large capacity.
Further, the laminated disk for a DVD is soft and thus tends to easily undergo warpage or bending because it is thinner than a conventional laminated disk such as a laser disk etc.
Furthermore, in the recording layer there occurs a trace of corrosion under the circumstances of high temperature and high humidity.
As a result of detailed examinations by the present inventors et al., it was estimated that the warpage or bending under the circumstances of high temperature and high humidity resulted from the inadequate thermal resistance and adhesiveness of the adhesive layer.
That is, under the circumstances of high temperature and high humidity, the adhesive layer generally requires thermal resistance. However, if thermal resistance is improved, the hardness of the adhesive layer is increased while its adhesiveness is lowered in many cases. In addition, if the moisture absorption property of the adhesive layer is high, the adhesive layer is easily swollen to lower adhesiveness. These reasons presumably cause the above distortion or warpage.
For the corrosion of the recording layer, it is estimated that because cationic polymerization makes use of an organic salt as a catalyst, such an ionic substance remaining in the adhesive layer causes the corrosion of a metal in an Al layer etc. forming the recording layer at high temperature and high humidity.
The present invention was made in view of the above-described problem, and a first object of the present invention is to provide a resin composition excellent in thermal resistance.
Further, a second object of the present invention is to provide an epoxy resin composition with adhesive properties which are not lowered even if the adhesive layer has increased thermal resistance and hardness or is placed under high humidity.
Moreover, a third object of the present invention is to provide an epoxy resin composition for preventing an adhered body from being subject to the adverse effect by an ionic substance in the adhesive layer even under high humidity circumstance.
Finally, a fourth object of the present invention is to provide an optical information recording medium e.g. optical disk excellent in reproduction of recorded signals, which is prepared by bonding disk substrates through an adhesive layer consisting of the resin composition as the first to third objects.